Fluid pressure operated clutch control systems for engaging and disengaging the driven member of a gear selecting transmission with a drive member such as a vehicular engine crank shaft, whether automatic or operator controlled, have been known for many years. Such systems are particularly advantageous when a source of fluid pressure already exists such as for example in motor vehicle applications where pressurized air or hydraulic fluid is used for braking purposes.
Generally such systems employ a transmission having an input or driven member that rotates when engaged with a drive member rotated by an engine crankshaft to enable the transmission to provide various gear ratio outputs to an output shaft such as a vehicle drive shaft. It is highly preferable to shift between gears of a transmission when the driven member of the transmission is disengaged from the engine drive member to prevent damage to the transmission gear train. Thus, the purpose of the clutch is to disengage the engine drive member from the transmission driven member while the gear shift is being made and, once made, re-engage the two together. Such can be accomplished in standard transmissions by depressing a clutch pedal and in automatic transmissions by use of engine speed sensors and other circuitry resulting in the automatic shift of gears at selected engine or transmission output speeds. Prior art clutches are normally either of the spring biased normally engaged or disengaged type. In certain situations, such as in automatic clutch systems for automatic mechanical transmission systems as illustrated in U.S. Pat. Nos. 4,081,065 and 4,361,060, the disclosures of which are hereby incorporated by reference, normally disengaged clutches may be preferable.
One of the potential problems with pressurized fluid type clutch systems utilizing normally disengaged clutches is their loss of function as a result of loss of fluid pressure for one reason or another for in most cases loss of fluid pressure will result in disengagement of the clutch which results in free-wheeling and loss of engine braking on the vehicle. Alternatively, loss of fluid pressure in a normally engaged fluid pressure clutch system to a value below a predetermined value may require hand disengagement by an operator for such systems in the past have not been provided with means for automatically disengaging the clutch.
Accordingly, it is highly desirable that fluid pressure controlled clutch systems utilizing normally disengaged clutches be provided with means of automatically insuring that the transmission becomes or remains engaged with the engine drive member when the fluid pressure drops below a predetermined minimum level and that normally engaged clutch systems are provided with means of automatically disengaging the clutch.